Melt butter. Mix with cocoa, sugar, milk and vanilla. Beat until smooth and
creamy. Add more powdered sugar if needed for spreading consistency. After
cake is frosted, decorate top with coarsely chopped walnuts. When I made
this, Paul and Monique really loved it. I used the 2/3 cup milk and omitted
the baking soda when I made it. We aren't too keen on yogurt.
Source: My Old Recipes